Electrode configuration for LIMCA

ABSTRACT

The present invention provides a method and apparatus for reducing electromagnetic noise pick up in a Liquid Metal Cleanliness Analyzer (LiMCA), used to detect and measure particles in molten metal. An first electrode inserted in the molten metal is electrically insulated from second and third electrodes, also inserted in the molten metal. Molten metal and particles pass between the first electrode and the second and third electrodes through a passage in the electrical insulation. The second and third electrodes have a configuration with respect to the first electrode sufficient to establish symmetrical current loops between the first electrode and the second and third electrodes when a current is supplied to the second and third electrodes. The current is supplied from an ultra-capacitor. Electromagnetic noise in the symmetrical current loops is detected and is added in opposition to reduce the amplitude of the electromagnetic noise.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 11/067,768filed Feb. 28, 2005, which claims the priority right of provisionalapplication No. 60/550,998 filed Mar. 4, 2004.

FIELD OF THE INVENTION

The invention generally relates to reducing electromagnetic noise pickup in an apparatus for detecting and measuring particles in moltenmetal.

BACKGROUND OF THE INVENTION

Molten metals, particularly molten aluminum and steel, are frequentlycontaminated by entrained non-metallic inclusions that give rise to avariety of shortcomings or defects in resulting products. Theseinclusions may cause the metal to tear during mechanical workingoperations, pin-holes and streaks in foils, surface defects and blistersin sheets, and increased rates of breakage during the production ofwire.

One analyzer that provides quick results and includes size andconcentration information of inclusions is a Liquid Metal CleanlinessAnalyzers (LiMCA). An LiMCA may comprise an electrically insulating wallmeans, often in the form of a sampling tube, having a small,precisely-dimensioned passage in a side wall. The tube is immersed inthe molten metal to be tested and a uniform stream thereof is passedthrough the passage while a substantially constant electric current isestablished through the stream between two electrodes disposedrespectively inside and outside the tube. The particles of interest havevery high resistivity compared to the molten metal and travel of aparticle through the passage is accompanied by a change in resistanceagainst the electric current producing an electrical pulse in thevoltage. The number of pulses produced while a fixed volume of metalpasses through the passage provides an indication of the number ofparticles per unit volume in the metal. Furthermore it is possible toanalyze the pulse shape to determine particle size and sizedistribution.

The LiMCA apparatus has been designed for “on-line” tests, to giveresults in seconds, but this often means that the apparatus is close tothe molten metal source and associated noise generating equipment.

Within the industrial environment of the LiMCA, there may be manysources of electrical or mechanical interference, or noise that affectthe results of electrical pulse detection. It is difficult in practiceto reliably extract the wanted pulse signals of the LiMCA system fromthese noise signals, since the noise may be of the same order ofmagnitude as the wanted pulse signals from detection of the smallerparticles. To this end, it has traditionally been preferred that thesupply current come from rechargeable batteries. Such batteries, forexample Nickel-Cadmium types, can be recharged at points during themeasurement cycle when data is not actually being collected but the useof batteries requires additional steps and equipment to ensure thatcharging is properly controlled. The batteries are also sensitive tohigh temperatures that can occur in the environment and are limited inthe number of charge-discharge cycles that can be applied. A vacuum orpressure source is generally used to move the metal through the passageand should be free of pump-generated pulses that may interfere with thesignal. The entire apparatus should also be shielded as much as possibleagainst outside electromagnetic interference.

The design and use of filters to reduce or eliminate interference is nowa well-developed art, but difficulties arise when used with LiMCAanalyzers, due to the relatively low voltage signal characteristic ofthe particle-indicating pulses and the fact that the pulse frequencies,corresponding to the number of particles per unit time passing throughthe passage, are of the same order of magnitude as those of many of theinterfering noise pulses. Shielding can be provided to reflect or absorbthe broadcast radiation before it reaches the apparatus but it isimpossible to achieve a perfect shielding because of the need for inputsand outputs to and from the system.

Typical LiMCA analyzers are described in U.S. Pat. Nos. 4,600,880,5,130,639, 4,555,662 and 5,039,935, herein incorporated by reference.U.S. Pat. No. 5,130,639 (Hachey) describes the use of variouscombinations of electrodes in a LiMCA analyser where the measurement andcurrent supply electrodes are separate and placed in noise reducingconfigurations. At least four, and up to six electrodes are required.

U.S. Patent Application No. 2002/0067143, also herein incorporated byreference, describes the use of ultra-capacitors as alternatives forbatteries in certain applications. Ultra-capacitors can be used as powersources in harsh environments because they are less temperaturesensitive than batteries and they can be rapidly charged and dischargedcompared to batteries, thereby requiring less control on thecharging-discharging process, and have very large cycle life compared tobatteries. However, ultra-capacitors have a lower volume charge densitythan rechargeable batteries and cannot therefore supply constant highcurrents for extended periods of time compared to batteries.

It is generally seen as desirable to develop an analyzer system that canreduce interference while simplifying design and requiring feweradditional parts.

It is further desirable to develop an analyzer system that can operatewith increased continuity and overcome the deficiencies of batteryoperation.

SUMMARY OF THE INVENTION

The present invention provides a method of reducing electromagneticnoise in wanted pulse signals produced by a LiMCA analyzer, having abody of molten metal containing particles, electrically insulating wallmeans having a passage formed therein for passage of the molten metal, afirst electrode inserted in the molten metal on one side of the wallmeans, a second and a third electrode inserted into the molten metal onan opposite side of the wall means to said first electrode and equallyspaced on either side of the first electrode such that the threeelectrodes fall in a common plane, said method comprising steps ofsupplying current equally to the second and third electrodes, saidcurrent passing with the molten metal through the passage to the firstelectrode to create symmetrical current loops between each of the secondand the third electrodes and the first electrode, which loops generatethe wanted pulse signals and are affected in an equal but oppositemanner by electromagnetic noise; and adding the wanted pulse signalsgenerated by each current loop to at least reduce the electromagneticnoise.

The present invention further provides a method of making nearlycontinuous measurements in a LiMCA analyzer, having a body of moltenmetal containing particles, electrically insulating wall means having apassage formed therein for passage of the molten metal, a firstelectrode inserted in the molten metal on one side of the wall means andat least one additional electrode inserted in the molten metal on anopposite side of the wall means to the first electrode and voltagerecording means connected between the first electrode and the at leastone additional electrode, said method comprising supplying current at aninitial pre-determined level to the first electrode and to the at leastone additional electrode to create a current loop between the at leastone additional electrode and the first electrode, said current loopgenerating a wanted pulse signal that is recorded by the voltagerecording means, wherein the said current is supplied from anultra-capacitor, monitoring the change in the current from saidultra-capacitor and providing a means for rapidly re-adjusting thecurrent output to the predetermined level while said voltage recordingmeans is temporarily prevented from recording the wanted pulse signal.

The present invention also provides an apparatus for reducingelectromagnetic noise in wanted pulse signals produced by a LiMCAanalyzer, having a body of molten metal containing particles,electrically insulating wall means having a passage formed therein forpassage of the molten metal and particles, an inside electrode insertedin the molten metal inside the wall means, and comprising a first and asecond outside electrode inserted into the molten metal outside the wallmeans on either side of the inside electrode, to form a common planewith the inside electrode, a current source for supplying currentequally to the first and the second outside electrodes, which currentpasses with the traveling molten metal through the passage to the insideelectrode to create symmetrical current loops between each of the firstand the second outside electrodes and the inside electrode that generatethe wanted pulse signals, detection means connected to the first and thesecond outside electrodes and the inside electrode, for detecting wantedpulse signals generated by the current loops due to travel of particlesthrough the passage, and for detecting electromagnetic noise thataffects equally but in an opposite manner each of the current loops; andamplifier means for adding the wanted pulse signals generated by eachcurrent loop to at least reduce the electromagnetic noise.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described in conjunction with thefollowing FIGURE:

FIG. 1 is a schematic view of an embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

In the present invention, symmetrical and stable currents fromelectrodes on the outside of a passage in an electrically insulatingwall to an electrode on the inside of the passage, together withsimultaneous symmetrical voltage pick up, have been found to self-cancelexternal electromagnetic noise sources if these sources equally affectboth current loops. For purposes of the present invention, a stablecurrent is considered to be one that is relatively smooth and free ofpulses.

FIG. 1 illustrates an LiMCA 10 according to an embodiment of the presentinvention. The LiMCA 10 comprises an insulating wall means in the formof a closed end sampling tube 16 immersed in molten metal 18 and sealedat its upper end to a mounting plate or tube (not shown), for example,by means of an elastomeric seal. Three transversely-spaced parallelelectrodes 12, 14 and 15 extend downwardly into the molten metal withelectrode 12 extending into the sampling tube 16 and electrodes 14 and15 extending directly into the molten metal 18.

The tube 16 is provided with an accurately-formed borehole or passage 22in a portion of its side wall. A line 24 disposed in the tube 16 isconnected to a vacuum source 26 to establish a vacuum in a cell formedby the tube 16. Molten metal 18 is drawn into the cell through thepassage 22 by the vacuum. Alternately, a pressure source 26 can be usedto expel molten metal from the cell via the line 24.

Measurements are taken as follows. Once the cell is immersed in moltenmetal, a vacuum is applied and molten metal is drawn at a controlledrate through the passage 22. A stable current, usually of the order of60-65 amps, is passed through the passage 22 by feeding equal currents(each being half the total current) to each of the electrodes 14 and 15,by way of current leads 30, 32. The current is returned via the singleelectrode 12 within the cell. The voltage is measured between each ofthe outer electrodes 14, 15 and the centre electrode 12 by means of apre-amplifier 40 of a design known to those skilled in the art. Thevoltage changes as particles pass through the passage giving rise tovoltage spikes. Once a predetermined amount of metal is drawn into thecell, sufficient to allow enough voltage spike measurements to be takento provide statistically meaningful results, the measurements arehalted, pressure is applied to eject the metal, and the process may berepeated.

The number of voltage spikes measured is used to determine the volumedensity of particles in the molten metal and the pulse shape may beanalysed to determine particle size distributions. The measurementperiod should be long enough to provide a sufficient sample size ofvoltage pulse measurements for a statistically meaningful measurement,and to increase the overall signal to noise ratio.

Whilst the current may be supplied by a battery arrangement, forexample, as described in U.S. Pat. No. 5,130,639, it is preferred thatthe current be supplied by a supply system which consists of anultra-capacitor 50 a current source 52 fixed shunt resistors 54,56, andvoltage controlled resistors 58,60. In use, the ultra-capacitor 50 isinitially charged or recharged while measurements are not being taken,by closing switches 62, 64. Charging is carried out to a predeterminedlevel, determined by the charging supply and capacitor specificationsand is typically 11000 Coulombs (amp-seconds). This charging/rechargingcan occur, for example, during the period of time that a sample of metalis being expelled from the cell. Once charged, the switches 62, 64 areopened. During the measurement period, discharge switches 66, 68 areclosed, and a current, controlled by the voltage on the capacitor 50 andthe value of the shunt and voltage controlled resistors 54, 56, 58 and60 (and measured, for example, by the voltage drop across the shuntresistors 54, 56) flows through the outer electrodes 14, 15, through thepassage and returns via the central electrode 12. The voltage controlledresistances are preset so that the current initially flows atpredetermined value, for example, 32.5 amps equally through each of thepair of resistances. During measurements, the current tends to decay.Because the volume charge density of the ultra-capacitor 50 isrelatively low compared to a battery, the decay is sufficiently fastthat the current would fall to an unacceptable level before themeasurement is complete. A too-low current would lead to the voltagepulse height becoming too low to provide a clear differentiation frombackground noise. To overcome this problem, when the current (asmeasured for example by the voltage drop across the shunt resistors 54,56) falls below a predetermined level, for example about 90% of theinitial current, the measurements are suspended and the voltagecontrolled resistors 58, 60 are adjusted to return the current to theinitial value. This process may be repeated several times during thecourse of one measurement, wherein a measurement is defined as the timeto draw the metal sample into the cell. The ultra-capacitor 50 itself isconveniently and rapidly recharged to its full charge during the step ofexpelling metal from the cell, during which time, measurements are nottaken. It should be noted that a current source based on anultra-capacitor 50 and as described as above may also be effectivelyused with other types of LiMCA apparatus, for example as described inU.S. Pat. Nos. 4,600,880, 5,130,639, 4,555,662 and 5,039,935, where acomparable current is required for measurements and the metal is drawninto and expelled from a similar cell.

Common sources of interference for the LiMCA 10 are induction furnaces,which broadcast punctual and continuous bursts of strong interferencethat are easily confused with the wanted pulse signals, thus filters,shields and insulation have limited efficiency. LiMCA analyzers are alsooften used directly in molten metal processing facilities and arelocated in close proximity to the induction furnace, well within thebroadcast area of the interference signal. The bursts of interferenceproduced by the induction furnace and also any mechanical vibrations arepicked up by the electrodes 12, 14, 15 in the form of interference, ornoise. While a signal pre-amplifier can amplify the wanted pulse signalsfor better reading, it cannot be located to receive the wanted signalswithout also receiving the interference.

All three electrodes 12, 14 and 15 may lie in the same plane. Theelectrodes 14 and 15 are disposed on the opposite side of the electrode12 and are equally spaced from the electrode 12 so that the electrodes14 and 15 interact in a similar manner with the externally generatedinterference to thereby cancel out the interference signal. The currentflowing through the outer electrodes 14 and 15 induces magnetic fieldsin each of the current loops. These magnetic fields are balanced,thereby minimizing the combined, detected effect of noise, generallycaused by vibrations, that is picked up by outside electrodes 14 and 15,which are equally exposed to magnetic fields. Also, balanced magneticfields minimize noise generation by the instrument itself resulting fromelectrical transients that arise when the measurement current is turnedon or off.

The signal, containing the wanted pulse signal detected between theelectrode 12 and the electrodes 14 and 15 is fed to detection means suchas a differential preamplifier circuit 40 to amplify the wanted pulsesignals while rejecting the equal and opposite electromagnetic noisesignals. From the differential preamplifier circuit 40 the signal isthen fed for further analysis and display to a computation and recordingapparatus 46. To further protect against noise, the preamplifier 40 andcomputing and recording apparatus 46 may be decoupled, for example byopto-couplers 44. The pre-amplifier 40 as well can be operated from arechargeable battery or more preferable from an ultra-capacitor (notshown), which can be recharged periodically from a disconnectable powersupply (also not shown).

An exemplary embodiment of such a computing and recording apparatus 46can be seen in U.S. Pat. Nos. 4,555,662 and 4,600,880 incorporatedherein by reference. The recording apparatus 46 takes the pulse signalsresulting from the travel of particles through the passage 22 in theinsulating wall 16 and produces a permanent visible record indicatingthe number of particles per unit volume of molten metal 18, theirindividual size if required, and their relative size distribution.

All critical electronics are preferably placed within a shieldingenclosure 70 to shield the electronics themselves from picking up noise.The shielding enclosure 70 can be connected as shown by the dottedconnection 80 to one of the two electrodes 14 or 15 to establish themolten metal as reference. Alternatively, for further noise reduction,the enclosure 70 may be connected to a separate (fourth) electrode, notshown, immersed in the molten metal in which case the connection 80 isremoved.

The electrode structures disclosed enable the LiMCA device to detectrelatively small size particles and, as discussed above, minimize noiseand interference to enable the relatively small, wanted signals to beadequately identified.

In the above embodiments of the invention, the molten metal may be anymolten metal, such as a flowing stream passing in a transfer trough.

Although in the above embodiments analyzing is carried out while avacuum 26 draws the molten metal 18 into the tube 16, it is alsopossible to test while the metal 18 is expelled from the tube 16 by aninternally applied pressure in which case the recharging of theultra-capacitor 50 would be carried out during the period the metal wasdrawn into the cell.

This detailed description of the apparatus is used to illustrate theprime embodiment of the system and the method of the present invention.It will be obvious to those skilled in the art that variousmodifications can be made in the present apparatus of the system andthat various alternative embodiments can be utilized. Therefore, it willbe recognized that various modifications can be made in both the methodand apparatus of the present invention and in the applications to whichthe method and system are applied without departing from the scope ofthe invention, which is limited only by the appended claims.

1. (canceled)
 2. A method of making substantially continuousmeasurements in a Liquid Metal Cleanliness Analyzer (LiMCA), having abody of molten metal containing particles, electrically insulating wallmeans having a passage formed therein for passage of the molten metal, afirst electrode inserted in the molten metal on one side of the wallmeans and at least one additional electrode inserted in the molten metalon an opposite side of the wall means to the first electrode and voltagerecording means connected between the first electrode and the at leastone additional electrode, said method comprising: a) supplying currentat an initial predetermined level to the first electrode and to the atleast one additional electrode to create a current loop between the atleast one additional electrode and the first electrode, said currentloop generating a wanted pulse signal that is recorded by the voltagerecording means, and wherein said current is supplied from a currentsource comprising an ultra-capacitor; b) monitoring the change in thecurrent from said ultra-capacitor; and c) providing a means for rapidlyre-adjusting the current output to said predetermined level while saidvoltage recording means is temporarily prevented from recording thewanted pulse signal. 3-5. (canceled)